Balun With Improved Common Mode Rejection Ratio

ABSTRACT

A balun includes a first winding which has a first terminal coupled to an input, and a second terminal coupled to a reference potential terminal. The balun includes a second winding magnetically coupled to the first winding. The second winding has a first terminal coupled to a first differential output, a second terminal coupled to a second differential output, and a tap coupled to the reference potential terminal. The balun includes a first parasitic capacitor which has a first terminal coupled to the first winding and a second terminal coupled to the second winding. The balun includes a third winding which has a first terminal coupled to the reference potential terminal and a floating second terminal. The balun includes a second parasitic capacitor which has a first terminal coupled to the third winding and a second terminal coupled to the second winding.

CROSS REFERENCE TO RELATED APPLICATION(S)

This application claims priority to India Provisional Application No.202041052242, filed Dec. 1, 2020, which application is incorporatedherein by reference in its entirety.

TECHNICAL FIELD

This description relates generally to balun transformers.

BACKGROUND

Balanced-to-unbalanced (balun) devices are transformers which arecommonly used in wireless communications to couple differentialradio-frequency, microwave, or millimeter-wave frequency signals betweenfunctional circuit blocks. Baluns are used for single-ended todifferential conversion or differential to single-ended conversion ofsignals.

In a single-ended to differential conversion, one of the two terminalsof a primary winding is grounded and the other terminal is excited witha single-ended input. The ideal resulting output on a secondary windingis purely differential (e.g., the potential is determined by thedifference between the two terminals of the secondary winding as opposedto the difference between one terminal and ground).

At high frequencies, the parasitic capacitance between the primary andsecondary windings leads to undesirable common-mode signals at thesecondary winding when the balun is excited with a single-ended input.The impedance of the parasitic capacitance becomes less at highfrequencies, which leads to capacitive coupling between the primary andthe secondary windings. The primary winding is asymmetrically grounded(e.g., one terminal is connected to ground while the other terminal isconnected to a power supply), but the secondary winding is magneticallycoupled to the primary winding, causing a degraded common-mode rejectiondue to this asymmetry.

Common-mode signals at the secondary winding terminals should besuppressed to maximize the signal power in the differential mode and toavoid common-mode variation in the operating point of subsequentcircuitry.

In a single-ended to differential conversion, the differential modeconversion gain is defined as the ratio of the differential signal powerat the secondary winding to the single-ended signal power at the firstterminal of the primary winding, where the second terminal of theprimary winding is grounded. The common mode conversion gain is definedas the ratio of the common mode signal power at the secondary winding tothe single-ended signal power at the first terminal of the primarywinding, where the second terminal of the primary winding is grounded.The common mode rejection ratio (CMRR) of the balun is defined as theratio of the differential mode conversion gain to the common modeconversion gain.

In a differential to single-ended conversion, the input at the primarywinding is not perfectly differential, which leads to a common modecomponent at the primary winding. The common mode component at theprimary winding is coupled to the secondary winding. The CMRR of thebalun is defined as the ratio of the differential mode conversion gainto the common mode conversion gain.

Maximizing the CMRR is desirable for conversion of the input signalpower to the desirable output signal power and also to reduce theundesirable common-mode output signal at the secondary winding.

SUMMARY

In one aspect, a balun includes a first winding which has a firstterminal coupled to an input, and a second terminal coupled to areference potential terminal. The balun includes a second windingmagnetically coupled to the first winding. The second winding has afirst terminal coupled to a first differential output, a second terminalcoupled to a second differential output, and a tap coupled to thereference potential terminal. The balun includes a first parasiticcapacitor which has a first terminal coupled to the first winding and asecond terminal coupled to the second winding. The balun includes athird winding magnetically coupled to the first winding. The thirdwinding includes a first terminal coupled to the reference potentialterminal and a floating second terminal. The balun includes a secondparasitic capacitor which has a first terminal coupled to the thirdwinding and a second terminal coupled to the second winding.

In an additional aspect, the tap is coupled to the center of the secondwinding.

In an additional aspect, the first and second parasitic capacitors haveequal capacitances.

In an additional aspect, the first terminal of the first winding iscoupled to an input voltage.

In an additional aspect, the first differential output provides a firstdifferential voltage and the second differential output provides asecond differential voltage.

In an additional aspect, a balun includes a first winding which has afirst terminal coupled to a first differential input, a second terminalcoupled to a second differential input, and tap coupled to a referencepotential terminal. The balun includes a second winding magneticallycoupled to the first winding. The second winding has a first terminalcoupled to a single-ended output and a second terminal coupled to thereference potential terminal. The balun includes a first parasiticcapacitor which has a first terminal coupled to the first winding and asecond terminal coupled to the second winding. The balun includes athird winding magnetically coupled to the first winding. The thirdwinding has a first terminal coupled to the reference potential terminaland a floating second terminal. The balun includes a second parasiticcapacitor which has a first terminal coupled to the third winding and asecond terminal coupled to the first winding.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of a single-ended to differential balun ofan example embodiment.

FIG. 2 is a common mode equivalent circuit of the balun of FIG. 1.

FIG. 3 is a circuit diagram of a differential to single-ended balun ofan example embodiment.

FIG. 4 is a common mode equivalent circuit of the balun of FIG. 3.

FIGS. 5A-5C are simulated waveforms of a balun of an example embodiment.

The same reference numbers or other reference designators are used inthe drawings to designate the same of similar (functionally and/orstructurally) features.

DETAILED DESCRIPTION

FIG. 1 is a single-ended to differential balun 100 (also referred to asa S2D balun transformer) of an example embodiment. The balun 100includes a first winding N1 (e.g., primary) which has a first terminal104 coupled to a single-ended input 106. The first winding N1 has asecond terminal terminal 108 coupled to a reference potential terminal110 (e.g., electrical ground). An AC voltage source Vin can be coupledbetween the single-ended input 106 and the reference potential terminal110.

The balun 100 includes a second winding N2 (e.g., secondary) which ismagnetically coupled to the first winding N1 (e.g., N1 and N2 may formthe primary winding and the secondary winding, respectively, of atransformer or N1 and N2 may be configured so as to facilitate magneticcoupling without forming a transformer). The second winding N2 has afirst terminal 120 which is coupled to a first differential output 122and has a second terminal 124 which is coupled to a second differentialoutput 126. The second winding N2 has a tap 128 (e.g., center tap) whichis coupled to the reference potential terminal 110. In some embodiments,the center tap 128 may be shorted to ground. In other embodiments, thecenter tap 128 can be an AC coupling capacitive element (not shown inFIG. 1) coupled to common potential (e.g. ground or electrical ground).A first resistive load RL/2 is coupled between the first differentialoutput 122 and ground, and a second resistive load RL/2 is coupledbetween the second differential output 126 and ground. Because, thebalun 100 provides a differential output, two equal loads (RL/2, RL/2)are shown coupled between the differential outputs 122 and 126. In someexample embodiments, a load (which may include a battery, circuitry,electronics or an electromechanical device) may be connected betweenterminals 122 and 126.

The first and second windings N1 and N2 may be in close proximity to oneanother such that the first and second windings N1 and N2 aremagnetically coupled. Due to close proximity, a parasitic capacitancerepresented by C₁₂ may be present between the first and second windingsN1 and N2. The parasitic capacitance C₁₂ degrades the CMRR of the balun100 by allowing common mode current to flow from the first winding N1 tothe second winding N2.

In an example embodiment, a third winding N3 (tertiary winding) is addedto the balun to reduce the effect of the common mode current flowingthrough the parasitic capacitance C₁₂ to the second winding N2. Thethird winding N3 has a first terminal 130 which is coupled to thereference potential terminal 110 (coupled, for example, to ground). Thethird winding N3 has a second terminal 132 which is floating (e.g. notconnected to another component or supply), as is discussed in furtherdetail below. The first and third windings N1 and N3 are magneticallycoupled.

The third winding N3 may be in close proximity to the second winding N2such that a parasitic capacitance represented by C₃₂ may be presentbetween the third and second windings N3 and N2. In an exampleembodiment, the capacitance C₃₂ may include a component which is addedintentionally. Thus, the capacitance C₃₂ may comprise a component addedintentionally and also a parasitic capacitance.

If the voltage Vin is applied to the single-ended input 106, a commonmode component Vin/2 is present across N1. The common mode componentVin/2 is equal to the average of the potential at the first terminal 104of N1 and the potential at the second terminal 108 of N1. Also,differential mode components which have equal amplitude, yet oppositepolarity, are present at the differential outputs 122 and 126. AssumingN1=N2, a differential mode component Vin/2 is present at the firstdifferential output 122 and a differential mode component −Vin/2 ispresent at the second differential output 122. Because the differentialmode components have the same amplitude, yet opposite polarity, thecenter tap 128 acts as a virtual ground, because it is a potential thatis half-way between the potential at node 120 and node 124.

Because the first terminal 130 of the third winding N3 is coupled to thereference potential terminal 110 (e.g., electrical ground) and thesecond terminal 132 of the third winding N3 is floating, a voltage whichhas equal magnitude of Vin but opposite polarity (i.e., −Vin) is presentat the second terminal 132. Thus, a common mode voltage −Vin/2 ispresent across the third winding N3.

FIG. 2 illustrates a common mode equivalent circuit 200 of the balun100. The equivalent circuit 200 includes the parasitic capacitor C₁₂which has a first terminal 204 coupled to a voltage source Vin/2 (commonmode component of Vin across N1). The equivalent circuit 200 includesthe parasitic capacitor C₃₂ which has a first terminal 206 coupled to avoltage source −Vin/2 (common mode component of Vin across N3). Thecapacitors C₁₂ and C₃₂ have respective second terminals 208 and 210which are coupled to an output terminal 212. A resistive loadrepresented by RL/4 (parallel equivalent of RL/2 and RL/2) is coupledbetween the output terminal 212 and ground. The common mode voltagesVin/2 and −Vin/2 are coupled to the secondary winding which leads to acommon mode voltage Vocm at the output terminal 212. If C₃₂=C₁₂, thecommon mode component of Vin/2 at the output terminal 212 isapproximately nullified by the common mode component of −Vin/2 at theoutput terminal 212, resulting in a low common mode gain. As a result,CMRR of the balun 100 is improved. In an example embodiment, theparasitic capacitance between magnetically-coupled windings is increasedor decreased by increasing or decreasing the insulation between thewindings or by increasing or decreasing the distance between thewindings.

FIG. 3 is a differential to single-ended balun 300 (also referred to asa D2S balun transformer) of an example embodiment. The balun 300includes a first winding N1 (e.g., primary) which has a first terminal304 coupled to a first differential input 306. The first winding N1 hasa second terminal terminal 308 coupled to a second differential input310. An AC voltage source Vin can be coupled between the first andsecond differential inputs 306 and 310.

The first winding N1 has a tap 312 (e.g., center tap) which, in someexample embodiments, may be coupled to a reference potential terminal314 (e.g., electrical ground). In other embodiments, the center tap 312may be an AC coupling capacitive element (not shown in FIG. 1) coupledto the reference potential terminal 314.

The balun 300 includes a second winding N2 (e.g., secondary) which ismagnetically coupled to the first winding N1 (e.g., N1 and N2 may formthe primary windings and secondary windings, respectively, of atransformer or N1 and N2 may be configured so as to facilitate magneticcoupling without forming a transformer). The second winding N2 has afirst terminal 320 coupled to a single-ended output 322. Second windingN2 has a second terminal 324 which is coupled to the reference potentialterminal 314 (e.g., electrical ground). A resistive load represented byRL is coupled between the single-ended output 322 and the referencepotential terminal 314. In some example embodiments, the load RL mayinclude a battery, circuitry, electronics or an electromechanicaldevice.

The first and second windings N1 and N2 may be in close proximity to oneanother such that the first and second windings N1 and N2 aremagnetically coupled. Due to close proximity, a parasitic capacitancerepresented by C₁₂ may be present between the first and second windingsN1 and N2. Because the voltage at primary winding N1 is not perfectlydifferential, a common mode component Vcm is present at the inputterminals 306 and 310. The common mode component Vcm at the inputterminals 306 and 308 is coupled to the output terminals 322 and 314,thus degrading the CMRR of the balun 300. Thus, it is desirable toreject the common mode component at the input terminals 306 and 310.

In an example embodiment, a third winding N3 (tertiary winding) is addedto the balun 300 to reduce the effect of common mode current flowingthrough the parasitic capacitance C₁₂. The third winding N3 has a firstterminal 330 which is coupled to the reference potential terminal 314.The third winding N3 has a second terminal 332 which is floating. Insome example embodiments, winding N3 is implemented as a continuation ofwinding N2 with terminal 330 being a tap off of the winding. In someexample embodiments, winding N3 is magnetically coupled to winding Ni.

The third winding N3 may be in close proximity to the first winding N1such that a parasitic capacitance represented by C₃₁ may be presentbetween the third and first windings N3 and N1, respectively. In anexample embodiment, the capacitance C₃₁ may include a component which isadded intentionally. Hence, C₃₁ may include a parasitic component and anintentionally-added component.

FIG. 4 is a common mode equivalent circuit 400 of the balun 300. Theequivalent circuit 400 has the parasitic capacitor C₁₂ which has a firstterminal 404 coupled to a common mode voltage Vcm (common mode componentpresent across N1). The parasitic capacitor C₁₂ has a second terminal406 coupled to the second winding N2.

The equivalent circuit 400 has the parasitic capacitor C₃₁ which has afirst terminal 408 coupled to Vcm. The parasitic capacitor C₃₁ has asecond terminal 410 coupled to the third winding N3.

The second winding N2 has a first terminal 420 which is coupled to asingle-ended output 422. The second winding N2 has a second terminal 424coupled to a reference potential terminal 426, which may be coupled to acommon potential (e.g. ground). A resistive load represented by RL iscoupled between the single-ended output 422 and a common potential(e.g., ground). The third windng N3 has a first terminal 428 coupled tothe reference potential terminal 426. The third winding N3 has a secondterminal 430 which is floating. A common mode current is injected intothe secondary winding from the primary winding through the capacitorC₁₂. The common mode current produces a common mode voltage Vocm at theoutput 422.

Because the second terminal 424 of N2 and the first terminal 428 of N3are both coupled to the reference potential 426 (e.g., electricalground) and the second terminal 430 of N3 is floating, the voltagepresent at the second terminal 430 of N3 is approximately equal to thevoltage present at the first terminal 420 of N1 but of an oppositepolarity (e.g. −Vocm). By selecting C₃₁ and C₁₂ to have equalcapacitances, the injected common mode current through C₁₂ is divertedaway from RL making the common mode output voltage Vocm of theequivalent circuit 400 to zero. Thus, by adding the third winding N3,the effect of the common mode current flowing through the capacitor C₁₂to the secondary winding N2 is reduced.

FIG. 5A is a simulated waveform of amplitude imbalance of a balun of anexample embodiment. In this example, the balun is designed for a 2.5 GHzlow noise amplifier (LNA). The x-axis represents frequency (GHz) and they-axis represents amplitude imbalance (dB). At 1.0 GHz, the amplitudeimbalance between two differential outputs is approximately −0.4 dB, andat 2.5 GHz, the amplitude imbalance between two differential outputs isapproximately −0.15 dB. Thus, the balun exhibits a very low amplitudeimbalance (i.e., less than 0.5 dB).

FIG. 5B is a simulated waveform illustrating phase imbalance of thebalun. The x-axis represents frequency (GHz) and the y-axis representsphase (degrees). At 1 GHz, the phase imbalance between two differentialoutputs is 2.66 degrees and at 2.5 GHz, the phase imbalance between twodifferential outputs is 2.98 degrees. Thus, the balun exhibits a verylow phase imbalance (i.e., less than 5 degrees).

FIG. 5C is a simulated waveform of CMRR of the balun. The x-axisrepresents frequency (GHz) and the y-axis represents CMRR (dB). At 1GHz, the CMRR is 33.2 dB and at 2.5 GHz, the CMRR is 32.12 dB. Thus,CMRR greater than 30 dB is obtained over a wide range of frequencies.

Windings N1, N2 and/or N3 may be implemented over a single semiconductorsubstrate in one or more example embodiments. For example, windings N1,N2 and/or N3 may be implemented as discussed in U.S. Pat. Nos.9,276,056B2 and 10,181,834B2 and United States Patent ApplicationPublication 2020/0388570A1 (also assigned to Texas InstrumentsIncorporated), all of which are hereby incorporated by reference intheir entirety.

In this description, the term “couple” may cover connections,communications, or signal paths that enable a functional relationshipconsistent with this description. For example, if device A provides asignal to control device B to perform an action, then: (a) in a firstexample, device A is coupled to device B; or (b) in a second example,device A is coupled to device B through intervening component C ifintervening component C does not substantially alter the functionalrelationship between device A and device B, such that device B iscontrolled by device A via the control signal provided by device A.Also, in this description, a device that is “configured to” perform atask or function may be configured (e.g., programmed and/or hardwired)at a time of manufacturing by a manufacturer to perform the functionand/or may be configurable (or reconfigurable) by a user aftermanufacturing to perform the function and/or other additional oralternative functions. The configuring may be through firmware and/orsoftware programming of the device, through a construction and/or layoutof hardware components and interconnections of the device, or acombination thereof. Furthermore, in this description, a circuit ordevice that includes certain components may instead be adapted to becoupled to those components to form the described circuitry or device.For example, a structure described as including one or moresemiconductor elements (such as transistors), one or more passiveelements (such as resistors, capacitors and/or inductors), and/or one ormore sources (such as voltage and/or current sources) may insteadinclude only the semiconductor elements within a single physical device(e.g., a semiconductor die and/or integrated circuit (IC) package) andmay be adapted to be coupled to at least some of the passive elementsand/or the sources to form the described structure either at a time ofmanufacture or after a time of manufacture, such as by an end-userand/or a third party.

As used herein, the terms “terminal”, “node”, “interconnection” and“pin” are used interchangeably. Unless specifically stated to thecontrary, these terms are generally used to mean an interconnectionbetween or a terminus of a device element, a circuit element, anintegrated circuit, a device or other electronics or semiconductorcomponent.

While certain transistors are described herein, other equivalent devicesmay be used in place of or in connection with these transistors. Forexample, in some embodiments, bipolar transistors, diodes, metal oxidesemiconductor field effect transistors may be used in place of or inconnection with the devices described herein. Furthermore, n-typedevices may be replaced with p-type devices and vice versa. While, insome example embodiments, certain elements may be included in anintegrated circuit while other elements are external to the integratedcircuit, in other example embodiments, additional or fewer features maybe incorporated into the integrated circuit. In addition, some or all ofthe features illustrated as being external to the integrated circuit maybe included in the integrated circuit and/or some features illustratedas being internal to the integrated circuit may be incorporated outsideof the integrated. As used herein, the term “integrated circuit” meansone or more circuits that are: (i) incorporated in/over a semiconductorsubstrate; (ii) incorporated in a single semiconductor package; (iii)incorporated into the same module; and/or (iv) incorporated in/on thesame printed circuit board.

While certain components may be described herein as being of aparticular process technology, these components may be exchanged forcomponents of other process technologies. Circuits described herein arereconfigurable to include the replaced components to providefunctionality at least partially similar to functionality availablebefore the component replacement. Components shown as resistors, unlessotherwise stated, are generally representative of any one or moreelements coupled in series and/or parallel to provide an amount ofimpedance represented by the shown resistor. For example, a resistor orcapacitor shown and described herein as a single component may insteadbe multiple resistors or capacitors, respectively, coupled in series orin parallel between the same two nodes as the single resistor orcapacitor. Also, uses of the phrase “ground terminal” in thisdescription include a chassis ground, an Earth ground, a floatingground, a virtual ground, a digital ground, a common ground, and/or anyother form of ground connection applicable to, or suitable for, theteachings of this description. Unless otherwise stated, “about”,“approximately”, or “substantially” preceding a value means ±10 percentof the stated value.

Modifications are possible in the described embodiments, and otherembodiments are possible, within the scope of the claims.

What is claimed is:
 1. A balun comprising: a first winding having afirst terminal coupled to an input, and having a second terminal coupledto a reference potential terminal; a second winding magnetically coupledto the first winding and having a first terminal coupled to a firstdifferential output, a second terminal coupled to a second differentialoutput, and a tap coupled to the reference potential terminal; and athird winding magnetically coupled to the first winding and having afirst terminal coupled to the reference potential and a floating secondterminal.
 2. The balun of claim 1, further comprising: a first parasiticcapacitor having a first terminal coupled to the first winding and asecond terminal coupled to the second winding; and a second parasiticcapacitor having a first terminal coupled to the third winding and asecond terminal coupled to the second winding.
 3. The balun of claim 1,wherein the tap is coupled to the center of the second winding.
 4. Thebalun of claim 1, wherein the first and second parasitic capacitors haveequal capacitances.
 5. The balun of claim 1, wherein the first terminalof the first winding is coupled to an input voltage.
 6. The balun ofclaim 1, wherein the first differential output provides a firstdifferential voltage and the second differential output provides asecond differential voltage. The balun of claim 1, wherein the balun isa single-ended to a differential balun.
 8. A balun comprising: a firstwinding having a first terminal coupled to a first differential input, asecond terminal coupled to a second differential input, and a tapcoupled to a reference potential terminal; a second winding magneticallycoupled to the first winding and having a first terminal coupled to asingle-ended output and a second terminal coupled to the referencepotential terminal; and a third winding magnetically coupled to thefirst winding and having a first terminal coupled to the referencepotential terminal and a floating second terminal.
 9. The balun of claim8, further comprising: a first parasitic capacitor having a firstterminal coupled to the first winding and a second terminal coupled tothe second winding; and a second parasitic capacitor having a firstterminal coupled to the third winding and a second terminal coupled tothe first winding.
 10. The balun of claim 8, wherein the tap is coupledto the center of the first winding.
 11. The balun of claim 8, whereinthe first parasitic capacitor and the second parasitic capacitor haveequal capacitances.
 12. The balun of claim 8, further comprising aninput voltage source coupled between the first and second differentialinputs.
 13. The balun of claim 8, wherein the output provides asingle-ended voltage.
 14. A balun comprising: a first winding having afirst terminal coupled to an input, and a second terminal coupled to areference potential terminal; a second winding magnetically coupled tothe first winding and having a first terminal coupled to a firstdifferential output, a second terminal coupled to a second differentialoutput, and a tap coupled to the reference potential terminal; a firstcapacitor having a first terminal coupled to the first winding and asecond terminal coupled to the second winding; a third windingmagnetically coupled to the first winding and having a first terminalcoupled to the reference potential terminal and a floating secondterminal; and a second capacitor having a capacitance approximatelyequal to the capacitance of the first capacitor having a first terminalcoupled to the third winding and a second terminal coupled to the secondwinding.
 15. The balun of claim 14, wherein the tap is coupled to thecenter of the second winding.
 16. The balun of claim 14, wherein thefirst differential output provides a first differential voltage and thesecond differential output provides a second differential voltage. 17.The balun of claim 14, wherein the first terminal of the first windingis coupled to an input voltage.
 18. A balun comprising: a first windinghaving a first terminal coupled to a first differential input, a secondterminal coupled to a second differential input, and tap coupled to areference potential terminal; a second winding magnetically coupled tothe first winding and having a first terminal coupled to a single-endedoutput and a second terminal coupled to the reference potentialterminal; a first capacitor having a first terminal coupled to the firstwinding and a second terminal coupled to the second winding; a thirdwinding magnetically coupled to the first winding and having a firstterminal coupled to the reference potential terminal and a floatingsecond terminal; and a second capacitor having a capacitanceapproximately equal to the capacitance of the first capacitor and havinga first terminal coupled to the third winding and a second terminalcoupled to the first winding.
 19. The balun of claim 18, furthercomprising an input voltage source coupled between the first and seconddifferential inputs.
 20. The balun of claim 18, wherein the outputprovides a single-ended voltage.